1. Technical Field
The present invention relates to a nitride semiconductor device, and more specifically to a nitride semiconductor heterojunction field effect transistor (FET), in which drain current is zero when gate voltage is not applied, as it is called a normally-off type.
2. Related Arts
A nitride semiconductor, such as GaN, AlGaN, InGaN, and the like, has greater band gap energy than the conventional semiconductors, such as Si, GaAs, and the like. Therefore, it is excellent as a semiconductor for high temperature or high breakdown voltage devices. In particular, using materials of these nitride semiconductors (GaN semiconductors), different from other wide band gap semiconductors, such as SiC or the like, it becomes possible to form a heterojunction such as AlGaN/GaN or the like. Therefore, a nitride semiconductor heterojunction field effect transistor (a heterojunction FET: HFET), or a high electron mobility transistor (HEMT) as another name, has been actively developed.
However, in an AlGaN/GaN heterojunction interface, due to a spontaneous polarization and a piezoelectric effect (piezo effect), there occurs positive electric charges at the AlGaN side. As a result, negative electric charges (electrons) are accumulated at the GaN side. As the accumulated electrons, without any doping for the AlGaN, forms a high-density two-dimensional electron gas, a resistance of a channel, that is to say an on-resistance of the HFET, becomes drastically smaller than the on-resistance of an AlGaAs/GaAs HFET.
Meanwhile, in regular devices used for power controlling, such as an inverter, a converter, and the like, a normally-off type FET, in which drain current is zero when a gate voltage is not applied, is used. However, in an AlGaN/GaN HFET, due to a spontaneous polarization and a piezoelectric effect, it is difficult to make the two-dimensional electron gas be zero, when gate voltage is not applied.
Regarding the AlGaN/GaN HFET, for obtaining a normally-off state (an enhancement mode), there is reported a method to reduce the polarization affect by forming the AlGaN layer thinner, such as disclosed in a document 1 (M. A. Khan et al., Applied Physics Letters, vol. 68, No. 4, January 1996, pp. 514-516). Moreover, for obtaining the normally-off type FET, there is reported a method to incorporate F− ions into the AlGaN layer by exposing a sample substrate in a CF4 plasma, and then to charge the AlGaN layer negative, such as disclosed in a document 2 (Yong Cai et al., IEEE Electron Device Letters, vol. 26, No. 7, JULY 2005, pp. 435-437).
In the former method disclosed in the above mentioned document 1, it is difficult to fabricate FETs with a complete normally-off state. Moreover, in this method, it is difficult to obtain a sufficient channel current, because a gate forward current is flowed under applying a positive bias onto the gate. Meanwhile, there is a method to suppress the gate forward current by adding a thin insulator of such as Al2O3 or the like onto the AlGaN. However, in the method, it is difficult to reduce an interface state of the Al2O3/AlGaN. Hence, electrons are trapped at the interface, and then channel charges are not able to be increased.
In the latter method disclosed in the above mentioned document 2, due to the CF4 plasma, the AlGaN layer is etched by the plasma, or it is damaged by the plasma. Hence, a controllability as a device becomes worse. For instance, it is difficult to control a threshold voltage in accurate, so as to make the drain current be zero, even when some amount of positive voltage is applied onto the gate.
In a non-volatile memory device using a silicon metal oxide semiconductor field effect transistor (Si-MOSFET), a polysilicon is deposited between a gate and an oxide film layer. And then by thermal oxidizing it, a floating gate is formed. Moreover, using a tunneling effect of the oxide layer, electrons are incorporated into or output from the floating gate, and then the threshold voltage is able to be controlled. Thus, zero, one, on, and off of the memory device is able to be identified.